you would need to rewrite the selection of maneuvers for Fighters, rework Maneuver Specialization, as well as change many levels of class features to detail the exact benefits you gain at certain levels for each style.
As you said, stunned is one of the top conditions to inflict on an enemy, other than
Dead, Unconscious, Dominated and Paralyzed
Anything that pulls away from there has to be seen as somewhat equal value.
Maybe they could lean into the broken condition for gear and allow a sword and board fighter to avoid melee weapon damage with their reaction in exchange for a broken shield. Maybe it only saves them from half damage when broken and is destroyed after doing that. Then CMs could improve this where they can defend against certain magic types. They not only hide behind a steel shield as well as a tower shield but take half damage from Dexterity save spells. If that cost exertion per use, it's not Evasion for the fighter without effort.
Single weapon fighting could give AC benefits. It's my opinion that later monsters have enough bonus to hit that they won't pick this later but it could be good at early levels.
Maybe their need to be more weapon specialties. Having said that, I think some need to consider the game instead of history. While it is accurate to say that people fought with pike and shield, they did it as a wall together with others. After having a player use this "exploit" for a campaign, I would see that removed. They got all the benefits of a two handed pole arm and a shield.
It is also about the DM talking to their players. I didn't know until we were several sessions into it that pike and shield were valid. At that point, he had done enough around it that to remove that would change his character, so I didn't. If the DM knows things like this, obviously state it up front. I might have been able to argue that the pike/shield didn't fit the character's concept but it could then be argued that given what was thrown at them, this was their adaptation to their adventuring career.
Aside: My group that played LU has two players that love 5E and one that didn't like PF1. We recently switched to try PF2 and get into it, especially with new Player Core, and the one who hated PF1 LOVES PF2. I think it's the three action economy, that is easier than the actions of 5E/LU, and the tactical choices that matter that he likes.